The performance of hydraulic systems at high temperatures is particularly sensitive to the properties of the hydraulic fluids. The effects of stability, viscosity, compressibility, volatility, gas solubility, and lubrication on hydraulic component and system performance are discussed. These properties singly and in combination affect many aspects of hydraulic system operation. The reaction of fluids to high temperatures causes significant changes in hydraulic performance.Presented as an American Society of Lubrication Engineers paper at the Lubrication Conference held in Rochester, New York, October 15–17, 1963.